It is known, as illustrated in U.S. Pat. No. 4,575,330 to C. W. Hull, to form three-dimensional objects of complex shape using ultraviolet light to solidify superposed layers of liquid ultraviolet-curable ethylenically unsaturated material at the surface of a liquid reservoir of such material. Thin walled objects are formed in this manner, apparently thick walls being hollow and dimensionally stabilized by thin internal webs. The ultraviolet dosage is limited to speed the action and to insure that only the thin lines struck by the laser beam on the surface of the unsaturated liquid will be solidified.
As will be evident, the thin walled object is incompletely cured and has inadequate strength and durability. Of particular significance is the fact that these stereolithographic processes are intended to form three-dimensional models which conform as accurately as possible with what was intended, this usually being set forth in drawings which are cross-sectioned by computer to guide a laser beam in the production of the superposed layers. However, the incompletely cured products tend to be somewhat distorted, so the accuracy of the model is less than desired.
As a result, it is necessary to provide relatively low viscosity flowable liquid compositions which cure rapidly and which exhibit decreased distortion when only partially cured to possess some limited "green strength".
It is desired to point out that the ultraviolet lasers used have limited capacity, so a larger exposure dosage would slow the stereolithographic process. Moreover, a greater exposure would increase the thickness and depth of the exposed line which solidifies, thus reducing the dimensional accuracy of the stereolithographic process.
More particularly, the objects under consideration are formed by the ultraviolet polymerization of liquid ultraviolet-curable ethylenically unsaturated material at the surface of a liquid reservoir of such material using an ultraviolet dosage in the range of about 0.1 to about 10, preferably about 0.2 to about 5, Joules per square centimeter which is sufficient to convert the unsaturated liquid into a lightly cross-linked solvent-swellable three-dimensional complexly-shaped polymeric thin-walled element constituted by heat-softenable solid polymer. A support is positioned beneath the surface of the reservoir to hold the polymerized layer which is formed. The liquid level is then raised, or the support lowered, and another layer is formed until the photoformed object is completed within the liquid reservoir.
The photoformed objects are thus only partially cured and are somewhat gelatinous and mechanically weak due to the low degree of cross-linking and the presence of unconverted monomers and oligomers (which are still unsaturated) within the partially polymerized polymeric structure of the solid object which is formed. The problem of this application is to have a low viscosity liquid composition which cures rapidly to speed the stereolithographic process and which, at the same time, exhibits minimal distortion in its partially cured condition so that the gelatinous and weak solid products produced by ultraviolet exposure will accurately reflect the shape of the object which it is desired to reproduce.
It is desired to point out that the distortion which is encountered is a composite of the extent of shrinkage encountered during the cure, the strength of the partially cured polymer structure, and whatever mechanical stresses are imposed on the product before the object removed from the reservoir is further cured to strengthen the same.
After the partially cured object has been withdrawn from the reservoir, one can proceed in various ways to more completely cure (thermoset) these gelatinous and weak objects after they have been removed from the liquid ultraviolet-curable ethylenically unsaturated material in which they were formed, and one must be careful that the additional cure does not introduce additional distortion. Nonetheless, if the incompletely cured object is distorted, that distortion is retained when the cure is completed, and it is the minimization of the distortion in the incompletely cured object which is withdrawn from the reservoir which is the objective of this invention.